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integration to achieve optimal results. Typically, for
adhesion and mechanical and thermal robustness, three-layer metallization schemes are used. A common scheme
is titanium/platinum/gold (Ti/Pt/Au), often used in
high-end devices because of its excellent stability and
endurance characteristics. However, because the thermal
conductivities of titanium and platinum are relatively
low, researchers sought, and found, an improved
material: chromium (Cr).
Chromium forms a carbide with diamond and
is also readily used as a barrier layer, enabling it to
perform both functions at a relatively high thermal
conductivity of Tc=93.9 W/mk. To test the thermal
effectiveness of chromium, samples were prepared at
the Centre for Device Thermography and Reliability
at Bristol University comparing a standard Ti/Pt/
Au (100/120/500nm) metallization with this novel
Cr/Au (100/500nm) configuration. Results showed
the effective thermal conductivity of the Cr/Au
metallization to be three to four times higher, compared
to Ti/Pt/Au.
To demonstrate the advantages of a Cr/Au
metallization compared to Ti/Pt/Au, high power
GaN-on-Silicon Carbide (SiC) high electron mobility
transistor (HEMT) devices were mounted to a CVD
diamond heat spreader. To ensure comparable results, all
samples were placed on a temperature stable platform
also made from high thermally conductive diamond
material. Results are shown in Figure 3. In the left
diagram, the base temperature is plotted for increasing
power output from the device. The temperature for
the Cr/Au configuration is significantly lower, at 9 W
device power output by about 10 °C. On the right hand
side, the graph shows the temperature as measured on
the transistor channel directly. In this case, the lower
thermal resistivity of the Cr/Au metallization decreases
the channel temperature by more than 20 degrees C at
9 W power output. The significantly lower temperature
will result in as much as a four times longer device
lifetime, or alternatively, will allow such devices to
be packaged smaller and with higher power densities,
according to a University of Bristol paper titled “
GaN-on-Diamond High-Electron-Mobility Transistor – Impact
of Contact and Transition Layers”.
GaN-on-diamond wafers: The next step in improved
device architecture
It’s clear, from extensive research and positive results
leveraging synthetic diamond heat spreaders, that there
is room to continue modifying device architecture for
improved performance. Engineers have long known the
potential of GaN to help create a new generation of
smaller and faster devices with greater power density.
Continued on page 19
Figure 3: Temperatures as a function of power for different
metallization schemed and solder thickness